Electrically small cavity antenna

ABSTRACT

A supersonic aircraft or missile broad bandwidth antenna is provided. This antenna is constructed into a cavity created in the fuselage or wing of the aircraft and covered with a radome for flush mounting. The cavity comprises side walls and a bottom constructed of electrically conductive materials which are caused to be electrically excited by an antenna element located within the cavity. This antenna element, which is capable of broad bandwidth operation, is either passively or actively tuned. Active tuning is carried out by a logic converter circuit connected to the antenna from a communications transceiver.

STATEMENT OF GOVERNMENT INTEREST

The invention described herein may be manufactured and used by or forthe Government of the United States of America for governmental purposeswithout the payment of any royalties thereon or therefor.

BACKGROUND OF THE INVENTION

This invention relates to radio antennae for use on aircraft, and inparticular relates to broad frequency bandwidth antennae for radiocommunications with subsonic and supersonic speed aircraft.

DESCRIPTION OF THE PRIOR ART

Communications antennae for use on aircraft have been the subject matterof patent protection issued in the prior art. Many antennae have beenmolded to lie flat on the surface of an aircraft.

Cary, U.S. Pat. No. 2,701,307, shows an aircraft radio antenna formed byflat metallic sheet portions mounted on the tail fin of an aircraft.These sheet portions form the poles or element portions of atransceiving antenna. Each element is insulated from the metallic skinof the aircraft by an insulating sheet of plastic. The design limitsitself to installations on a vertical stabilizer. Moreover, there is noindication that continuous broad-band performance is obtained.

Other types of VHF and UHF broad band antennae have been designed as airfoil type, blade-shaped antennae. These antennae are designed to have astructure blade member attached to the surface of the aircraft andextending outwardly therefrom. A blade antenna structure is intended tobe mounted externally on the fuselage or wings of an aircraft.

The antenna element portions of such external blade antennae have beenformed as part of the blade-shaped structure to lie flat on the surfaceof each blade and to be electrically insulated from the metal structureby an insulating member.

Young et al., U.S. Pat. No. 3,220,006, show such an external bladeantenna where the entire metal skin of the blade structure forms theelement portions of the antenna itself. In the Young structure, theblade is hollow and filled with a structurally capable dielectric.Portions of the blade skin are physically and electrically separatedfrom one another to form the separate elements of the antenna. Alaminated dielectric structural material, such as fiberglas sheet,connects the metal portions of the skin to present an aerodynamicallysatisfactory surface. These are mechanical improvements to a groundplane VHF antenna where a blade-type dipole configuration is obtained byreflecting a monopole element in a ground plane.

Dolan, U.S. Pat. No. 3,453,628, shows a silicon-based adhesive formounting his antenna elements to an external blade structure. Theadhesive acts as a vicious damping medium to reduce aircraft vibrationsto the laminated skin antenna elements glued to the flat surfaces of theblade and electrically insulated therefrom.

Anderson et al., U.S. Pat. No. 3,210,764, show a dual band antennamounted on the surface of an external standard blade structure. Aplurality of flat film-type elements are attached to various surfaces ofthe blade structure. A first element acts as a monopole radiatingelement on one side of the blade and also serves as a ground plane forother elements positioned on the opposite side of the blade. Theopposite side of the blade carries separate UHF and VHF antennaelements. Anderson, et al. show a standard blade antenna used for narrowband VHF and UHF communications with a built in passive matching networkand a band separation filter.

Demko, U.S. Pat. No. 4,072,952, shows an external blade antenna withfilm-type antenna elements on the surface of the blade. Demko addsmicrowave landing system (MLS) antenna elements onto the blade structurewhich had previously contained only VHF/UHF antenna elements.

A "Tee" slot blade antenna, where the metallic skin of a blade structuremounted externally to the fuselage of an aircraft forms the elements ofa VHF antenna, is shown by Robin et al., U.S. Pat. No. 4,509,053. LikeYoung et al. above, Robin et al. uses a structural dielectric tomaintain the blade shape. An aerodynamically continuous surface of theblade is assured by dielectric laminated fiberglas sheets fillingbetween skin antenna elements. The resonant frequency of the antenna isdetermined by the lengths and relative positions of longitudinal andtransverse dielectric sections which determine the remaining antennaelement real estate. Impedance matching circuits are incorporated toelectrically tune the antenna.

The use of electrical circuits to control the tuning of external bladeantenna elements is also shown by Sawicki et al., U.S. Pat. No.4,087,817. A simple blade shaped monopole element has a molded coveringof dielectric material. A loop antenna element is mountedperpendicularly to the blade antenna. An electrical network is utilizedto couple the blade and loop antennae to an ADF (automatic directionfinder) receiver and a VHF communications transceiver.

The aerodynamic performance of very high speed and ultra high speedaircraft and missiles can be affected by external blade antenna. Fluiddynamic disturbances set up by external blade antennae are often timesundesirable. Vibrations in an aircraft or missile can alter antennaperformance and even destroy the antenna. Heat due to friction can alsoalter or harm sensitive antenna elements.

Cavity antennae and flush mounted antennae have been considered as asolution to these problems. By mounting a communications or microwaveantenna within a cavity in an aircraft/missile body, there is created anopportunity for alternate antenna element structures. With cavityantennae the constraints for flat film or skin surface elements, as usedwith external blade antennae, are removed. Cavity type designs alsoafford more protection of antenna elements.

Stang, U.S. Pat. No. 3,725,941 shows a high frequency, high power coiltype transmitting antenna embedded in a dielectric notch shapedenclosure which is held in a structural box member formed as an integralpart of the leading edge of the vertical stabilizer of an aerospacevehicle. The dielectric fill material conforms to the shape of thestabilizer and is of a size to cooperate with signal modification by themetallic structure of the stabilizer. The antenna is very narrow banded,single frequency, and relies upon the fuselage to become excited andenhance the antenna's efficiency. It can be manually tuned to differentfrequencies by adjustment of capacitors or inductors.

A subsurface fuselage antenna is shown by Milligan, U.S. Pat. No.4,431,996. Milligan makes a narrow circumferential aperture around amissile body. This slot in the skin of the missile holds four separateantenna segments, each being an annularly shaped quarter wavelengthstandard microwave antenna. A dielectric structure, such as a radomecovers the slot. Although this structure has multi-frequency capability,it requires a separate antenna for each frequency and does not havegeneral broad band capabilities.

Howell, U.S. Pat. No. 3,403,403, shows a filter assembly for a missilenose cone. An antenna is located within the nose cone and radiatessignals through an elongated slot. A printed circuit plural boardassembly forms a filter for unwanted signals. The design allows 245.3MHz telemetry signals to radiate from the antenna. The filter operatesat 1300MHz, only.

Eng et al., U.S. Pat. No. 4,245,222, shows a dual function antenna in asingle slot-type cavity near the base of a missile body. The slot shapedcavity has a pair of parallel ports, one each for each of twotransversely positioned half wavelength antenna elements used fortelemetry and radar bands. The size of each port is tuned inrelationship to the wavelength of its respective antenna and thereforethe structure is focused for a specific bandwidth.

Frosch, U.S. Pat. No. 4,287,518, shows a flush mount, rectangular boxcavity, micro-strip dipole antenna. A pair of mutually orthogonaldielectric plane surfaces, mounted normal to the plane of an open faceof the box cavity, each carries printed antenna dipole elements. Aradome member may cover the open face. The positioned antenna dipoleelements are removed from the radome surface and the heat distortiongenerated therefrom. The antenna cavity must be electrically resonantand passively matched thereby it is physically large. The design is1.5:1 in bandwidth.

Blasko, U.S. Pat. No. 3,613,098, shows a small cavity VHF antennaadapted for flush mounting in an aircraft vertical stabilizer. Thecavity extends completely through the stabilizer and contains a signalemanating structure. A radome covers each side of the cavity to providea continuous stabilizer skin. The emanating structure contains arectangular cone shaped reflector structure opening towards the rear ofthe stabilizer with a signal conductor at its tip/point and back platebehind the conductor. The cavity and its enclosed emanating structureare balanced so as to radiate from both the port and starboard sides ofthe stabilizer thereby creating an omnidirectional pattern in azimuth.The design is inherently narrow banded VHF (118-136 MHz), the cavity andrectangular cone shape would accentuate the return of a radar signal.

This prior art is either directed to broad band external blade antennaeof the size large enough to be restrictive for use on supersonicaircraft and missiles or of cavity antenna designed for specific narrowband performance.

The recent development of broad frequency band communication radios,i.e., 30-400 MHz, has necessitated the development of blade antennae ofcomparable bandwidth. Since, in many applications the system is to beinstalled on high subsonic or supersonic aircraft, the blade antennasize is restricted to heights, typically, of 14.5 inches and 9 inches,respectively. Although the passively matched 14.5 inch height bladeantenna has performed adequately over the 30-400 MHz band, the 9 inchheight blade has had unsatisfactory performance especially in the 30-88MHz portion of the band due to its small aperture height. Several bladeantennae now available commercially overcame this low gain problem byincorporating an actively tuned aperture thereby increasing the gain atthe lower frequencies to be comparable to or greater than that of the14.5 inch height blade antenna with passive impedance matching.

However, communications requirements in recent years together with theenhanced aerodynamical requirements on the aircraft, have provided aneed for extremely broad band flush mounted antenna apertures.Applications on the bottom of the fuselage of aircraft with littleground clearance offer additional uses. It is therefore important thatthis antenna provides an electrically small image while being comparablein performance to standard fuselage mounted blade antennae that aresuitable for long range system operation.

Additionally, the antenna aperture should be of small physical size inrelation to the frequency of operation in order to make it suitable forinstallation on high performance aircraft with a minimum of availablespace.

SUMMARY OF THE INVENTION

An object of this invention is to provide an extremely broad bandwidthflush mounted aircraft or missile antenna of comparable performance to astandard fuselage mounted blade antenna which is suitable for long rangesystem operation.

A second object of the present invention is to provide such a flushmounted antenna with an electrically small cavity in relation to thefrequency of operation of the antenna.

A further object of the present invention is to provide such an antennacavity whose aperture is excited by an element located internal to thecavity.

The objects of the present invention are realized in an electricallysmall cavity antenna mounted in the fuselage or other structural memberof an aircraft to provide an aerodynamic flush mounting. A cavity isformed in the fuselage with four walls and a bottom of conductivematerial. The cavity wall shape need not be square or rectangular,however, the modification of this wall shape will affect the performanceof the antenna.

An antenna element is used to excite the cavity. This antenna element isa blade-type element which is capable of broad bandwidth performance andcan either be passive or actively tuned. The active antenna element istypically tuned in frequency from the serial data generated by acommunications transceiver through a transducer mechanism commonly knownas a logic converter.

This transducer accepts serial data from the transceiver, or directlyfrom a data bus, and through appropriate output line drivers activatesswitches, either internal to the antenna element or nearby, such thatthe proper coils, capacitors and required impedance matching componentscan be inserted in the circuit to resonate the antenna element in thecavity at each selected frequency. A commercially available tuner may beused as the tuning speed of some commercially available tuners is morethan adequate to handle the required "hopping" rates of today'savailable transceivers.

Lightning suppressers are also included in the antenna element tominimize the effects of lightning entering the aircraft. The possibilityof lightning strikes is greatly minimized with the flush mounted antennadesign as compared to an external blade antenna design.

The cavity forms a well in the skin of the aircraft. The open face oraperture of the cavity can be suitably enclosed with a radome of lowloss characteristics such as Teflon or fiberglas.

DESCRIPTION OF THE DRAWINGS

The features, advantages and operation of the present invention will bebetter understood from a reading of the following Detailed Descriptionof the Invention in conjunction with the following drawings in whichlike numerals refer to like elements and in which:

FIG. 1 is a perspective view of the electrically small cavity antenna ofthe present invention mounted in the fuselage of an aircraft;

FIG. 2 is an expanded perspective view of the electrically small cavityantenna of FIG. 1;

FIG. 3 is an expanded perspective view of an alternate embodiment of theelectrically small cavity antenna of FIG. 2 showing the antenna elementin a side mount position;

FIG. 4 is a block diagram of an electrical circuit for tuning theperformance of the antenna element for optimum voltage standing waveratio;

FIG. 5 is a block diagram of the electrical test set up for optimizationof gain of the antenna element;

FIG. 6a is a block diagram illustrating the transceiver and logicconverter circuit connection to the antenna element;

FIG. 6b is a block diagram illustrating the communications signaloperation of the logic converter to antenna element connection; and

FIGS. 7 through 11 show a plan view (designated "a") and two sectionalside views (designated "b" and "c", respectively) of various cavityconfigurations and antenna element placements.

DETAILED DESCRIPTION OF THE INVENTION

An electrically small (i.e. substantially 1/50th of a wavelength incross-section of diameter opening) cavity antenna 10 is intended to bemounted in the fuselage 11 of an aircraft, FIG. 1. This antenna issuitable for operation over the broadband 30-400 MHz frequency band. Itis a flush mounted antenna which radiates or receives verticallypolarized waves over that continuous frequency band.

FIG. 2 shows an expanded perspective view of the flush mountedembodiment shown in FIG. 1. The fuselage 11 can be cut out to fit theantenna 10 at any space available in the airframe, typically, on the topand/or the bottom of the fuselage 11. Sites are chosen to provide thebest "look angle" or field of view of the antenna similar to that chosenfor a standard blade antenna installation. The installation can includea flush mount finish surface 13 through which the cavity opens. Thecavity is comprised of side walls 15 and a bottom wall 17. A blade-typeantenna element 19 is positioned in the center of the bottom wall 17 toextend outwardly in parallel to the side walls 15. This blade antennaelement 19 will be discussed further below.

As shown in FIG. 2, the cavity defined by the walls 15 and the bottom 17is rectangular in shape. In this embodiment, the width of theelectrically small cavity is approximately 22 inches (0.06 wavelengths)and the length of the cavity is approximately 24 inches (0.06wavelengths), while the depth of the cavity is approximately 10 inches(0.03 wavelengths). The walls 15 and the bottom 17 are formed ofaluminum sheet or any other comparable conductive material. This cavitywall shape need not be square or rectangular, but can also be circularin construction or have tapered walls, as will be discussed furtherbelow. The open aperture of the cavity can be suitably enclosed with aradome 20.

FIG. 3 shows an alternate position embodiment for the blade-type antennaelement 19a. Here, the blade antenna element 19a is mounted on a sidewall 15.

The antenna elements 19, 19a which are used to excite the cavity wallscan be of either passive or active tunable designs. The best efficiencyand highest gain are achieved with actively tuned blade antennaelements. The conductive walls of the cavity and blade act as radiatingelements.

To ensure adequate protection from the environments, the cavity can becompletely sealed to prevent the intrusion of water and/or moisture. Toadequately provide against vast pressure change differentials, which mayresult in radome failure, the cavity can also be filled with low lossfoam 22, typically of the 2 pounds per cubic foot density. The radome 20is then attached to the foam 22 and to the finish surface 13 with aproper dielectric adhesive.

The blade antenna element 19, 19a, can be of a commercially availabletype as offered by Chelton Electrostatics, Ltd., Marlow Buckinghamshire,England, model type 12-190 series or equivalent. This blade antenna isactively tuned and provides VHF/UHF performance in a frequency range of30-400 MHz. Its Voltage Standing Wave Ratio (VSWR) for VHF processing is2.5:1 maximum, and for UHF processing is 2.0:1 maximum. The blade isapproximately 9 inches high and 9 inches long at its base, with anelement tube on its top edge which is approximately 11 inches longdependent on antenna type.

A series of printed circuit inductors are used to resonate a capacitiveradiating element in this Chelton antenna 19, 19a to the requiredfrequency of operation. The switching of the inductors in and out ofcircuits is accomplished by P-I-N diodes that are themselves controlledby voltage levels received from a separate logic converter. Frequencysetting information is initially derived from an associated transceiverand then translated into the correct antenna tuning code by the logicconverter. This will be discussed further below.

FIG. 4 shows a block diagram of an electrical circuit for optimizing theperformance of the blade antenna element 19 under manual operation. Atransmitting source 21, typically implemented by a frequency generator,is connected into a VSWR measuring device 23. This VSWR measuring device23 is connected through a first shielded coaxial cable 25 to the bladeantenna element 19 mounted in its associated cavity and ground plane. Amanual toggle switch box circuit 27 is also connected to the bladeantenna element 19 through a second shielded cable 29. The manual toggleswitch box 27 is powered from a multi-conductor DC power supply 31 whichis connected thereto.

After the test equipment is set up as shown in FIG. 4, the transmittingsource 21 is set to transmit a 30 MHz signal and all toggle switches inthe manual toggle switch box are set to the off position. The manualtoggle switch box 27 is conveniently set up as typically an 8 switch or8 position (instruction) device dependent on antenna type.

With the transmitting source 21 set at the first frequency value, theVSWR value from the measuring device 23 is recorded. This process istypically repeated every 2 MHz from 30 MHz to 400 MHz. As part of thisprocess, the 256 possible combination switch positions for the manualtoggle switch box 27 are successively set and the VSWR value for thatsetting is recorded. In this manner, switch settings for the toggle box27 are established to offer VSWR values of less than 2.5:1 at eachmeasured frequency.

The optimization of the gain of the blade antenna element 19 can beestablished as shown in the electrical test set up of FIG. 5. Atransmitting source 21a is connected through a third coaxial shieldedcable 25a to a 30-400 MHz transmitting antenna 33. This antenna 33 ismounted at a height 35 above the ground on an antenna tower 37. Forpurposes of the test at hand, the height 35 is set at approximately 43feet.

A standard gain reference antenna 24 and the blade antenna element 19are positioned at a reference distance 39 from the transmitting antenna33. For the purposes of this test, the distance 39 is set atapproximately 100 feet.

First, the recessed blade antenna element 19 in its associated cavity isset on a turnable antenna stand 41 with a 32 foot diameter ground plane.The antenna stand 41 is positioned on top of a building 43 or otherconvenient structure.

A manual toggle switch box 27a provides typically eight control lines 45to the blade antenna element 19 for tuning the frequency of this element19. This manual toggle switch box 27a receives power from a DC powersupply 31a. A radio frequency receiver 47 is connected to the bladeantenna element 19 through a fourth coaxial shielded cable 49.

This test set up is used to empirically determine the switch positionsof the manual toggle switch box 27a for the maximum gain values of theblade antenna element 19. The transmitter source 21a is set to transmita frequency of 30 MHz and all the toggle switches on the switch box 27aare set to the off position. The received power level as sensed by thereceiver 47 is recorded in decibels (db).

As with the previous procedure, these steps are repeated for every 2 MHzfrom 30 MHz to 400 MHz. Additionally, for each frequency tested, thesequence of all of the successive combinations of the switch box 27a aretested and the amplitude value is recorded for each frequency. Thisdata, therefore, will provide a list of values for switch positions thatoffer the highest received power level at each measurement frequency.

The blade antenna element 19 can be replaced with a standard gainreference antenna 24. A comparison of the values of the respective powerlevels for each frequency for the blade antenna element 19 with respectto the standard reference antenna can be made.

From the information obtained in the VSWR and gain tests, a single listof switch positions for each frequency is obtained that offers optimumantenna performance within certain instances a compromise between bestgain and best VSWR.

A communication transceiver system hook up for the flush mounted tunablecavity antenna 10 is shown in FIG. 6a. A transceiver radio 51 isconnected through cabling 53 into a logic converter circuit 55. Thislogic converter circuit 55 provides an output on the control line 57 totune the blade antenna element 19. This blade antenna element 19 may beon a fixed position, as shown in the cavity embodiments of FIGS. 1 and2. The transceiver radio 51 is connected through a RF frequencytransmission line 61 to the blade antenna element 19.

The connection 53 of FIG. 6a carries serial data information 63 from thetransceiver 51, as shown in FIG. 6b. This serial data 63 carries a codefor one of the frequencies that the transceiver 51 may be operating atthe moment. As an example, an instruction word 65 would be present whena 30 MHz signal is being transmitted by the transceiver 51. This wouldbe likewise true when the transceiver 51 is switched into the receivemode. Other individual instruction words 67 through 69 are present, onan exclusive basis, as the transceiver 51 is set for other frequenciesup through its maximum frequency of 400 MHz.

These instruction words 65, 67, 69 are connected into a PROM 73 andtiming and control circuits 75 through a connection line 71. The PROM 73has recorded, i.e. stored, in it the optimum or desired toggle switchsettings for the switch boxes 27, 27a for yielding the desired VSWRreadings and the maximum gain values obtained empirically with the testset ups of FIGS. 4 and 5. This is a control "profile" for tuning theparticular blade antenna element 19 for a particular operatingfrequency.

The PROM 73 provides an instruction word to control circuits 75 as afunction of the frequency 65, 67, 69 of operation of the transceiver 51.The PROM 73, associated control circuits 75 and line drivers 79 arelocated within the logic converter 55. The output of the controlcircuits 75 and prom(s) 73 is carried on individual lines 77implementing the structure previously discussed with respect to thecontrol lines 45 of FIG. 4 and control lines 29 of FIG. 5. Each of thelines 77 in FIG. 6b has an associated line driver 79 to increase thepower and assure proper signal levels. These control lines are thenconnected to tune the operation of the Chelton Electrostatics, Ltd.blade antenna or equivalent according to the manufacturer'sspecifications.

The signals out of the line drivers 79 automatically simulate the manualoperation of the toggle switch box to emulate the switch positions 81and to tune the blade antenna element 19 to any of the instantaneousfrequency 83, 85, 87 of the transceiver radio 51.

The data for tuning the blade antenna element 19 will vary with thechoice of available blade antennae. The frequency setting information isusually derived from the associated transceiver 51 and translated intothe correct antenna tuning code 81 by the PROM 73 and control circuits75 within the logic converter 55.

To determine the proper tuning code of another blade antenna, this bladeantenna with its associated cavity would likewise have to be mounted ontop of a ground plane in a free-space environment. Again, a manuallyoperated switch box consisting of typically eight or more toggleswitches, depending upon the blade antenna model selected, would then beconnected to the antenna. Appropriate D-C voltage levels are applied tothe switches depending upon the antenna model. A signal source is thenconnected to a transmitter antenna at an appropriate distance from thetunable blade antenna mounted on the ground plane.

The signal source is then set to the first frequency and the tunableblade antenna is connected to a suitable receiver with the mechanicaltoggle switches moved individually in a systematic order to obtain thebest gain from the blade antenna. All combinations of the toggleswitches are worked through with every frequency for mapping thefrequency band between the minimum and maximum frequencies of operation.The switch positions offering the best gain are noted and recorded foreach frequency. A determination of absolute gain is made with respect toa reference antenna.

While this procedure has been described as being manually establishedand mapped, the transmitting of various frequencies can be computerizedand automatically sequentially walked through. Likewise, the sequentialsetting of the control switch lines by means of the switch box 27, 27acan be computerized and automatically conducted. Computer programmableestablished control operations can therefore map the data necessary fora PROM 73 "profile" for a selected blade antenna 19.

Likewise, the VSWR measurements for the tunable blade antenna will ofnecessity be repeated as the selection of a blade antenna element 19 ischanged. The VSWR measurement test procedure can also be automated witha computer program control procedure.

While the first and second embodiments shown in FIGS. 2 and 3 utilize arectangular conductive cavity defined by the walls 15 and the bottom 17,other cavity shapes can be used for the present invention withoutdeparting from the intent and scope thereof. FIGS. 7 through 11 show aplan view and two sectional side views of various cavity wallconfigurations and antenna element placements. Of these, FIG. 7 is therectangular cavity wall embodiment described above in connection withFIG. 2 and shown here for reference to the other cavity configurations.

A horn shaped cavity is shown in FIG. 8. Here each of the side walls 15slant inwardly toward a very small bottom wall which is large enough tosupport the blade antenna element 19. In this embodiment, FIG. 8, theside walls slant outwardly and continue to the edge of the cavity.

FIG. 9 shows a narrowed shaped horn cavity wall structure where the horncreated by the cavity walls is more elongate. FIG. 10 shows arectangular cavity, similar to the cavity of FIG. 7, but with thecorners of the cavity blocked off to form octagon shaped side walls.Lastly, FIG. 11 shows a cylindrically shaped cavity.

Each of the cavity configurations shown in FIGS. 7 through 11 wereevaluated according to the above described test procedure forperformance over the frequency band range of 30-400 MHz. For this testevaluation, the cavity size varied although the outside dimensions ofthe cavity support structure were held constant.

Of the various configurations evaluated the design shown in FIG. 7 (thereference configuration) offered the best performance. This was duemainly to the larger volume of cavity area around the blade antennatherefore allowing it to radiate from a larger aperture. Theconfiguration of FIG. 8 offered somewhat comparable performance in the30-400 MHz frequency band, where performance is measured as frequencyresponse, however, it never achieved full efficiency, measured as poweroutput, as the reference configuration of FIG. 7.

The configuration of FIG. 9 offered comparable gain in the 30-60 MHzfrequency region but fell off as much as 5 db in the frequency band of150-400 MHz from the configuration of 7. The configuration of FIG. 10performed closely to that of FIG. 9, while the performance of theconfiguration of 11 was considerably worse than all of the otherconfigurations. In frequency band by approximately 5 db for thisconfiguration with respect to the reference configuration of FIG. 7.

Changes can be made in the above-described invention without departingfrom the intent and scope thereof. It is therefore intended that theabove-description be read as illustrative of the invention and not beinterpreted in the limiting sense.

What is claimed is:
 1. An electrically small cavity, broad bandwidthcommunications antenna for aircraft, comprising:an electrically smallconductive cavity mounted within a structural member of said aircraft,said cavity including a conductive enclosure which opens outwardly andwhich terminates at the surface of same aircraft structural member; adielectric material covering said open face of said cavity and providingan aerodynamically flush mount for said antenna; a broad bandwidthantenna element positioned within said conductive enclosure andoperating to excite same; and a tuning circuit connected to said antennaelement which tunes said antenna for optimum radiation efficiency so asto continuously operate over the full bandwidth of 30 to 400 MHz.
 2. Thecavity antenna of claim 1 wherein said antenna element is connected to acommunications transceiver and wherein said tuning circuit includes alogic converter connected to said communications transceiver and saidantenna element.
 3. The cavity antenna of claim 2 wherein said antennaelement is a blade type antenna element.
 4. The cavity antenna of claim3 wherein said blade type antenna element is a tunable, hi-gain,broadband blade antenna element.
 5. The cavity antenna of claim 4wherein said conductive enclosure is electrically excitable by saidblade antenna element to operate as a radiating element.
 6. The cavityantenna of claim 5 wherein said conductive enclosure includes conductiveside walls and a conductive bottom wall connected thereto.
 7. The cavityantenna of claim 6 wherein said conductive side and bottom walls form arectangular box structure.
 8. The cavity antenna of claim 7 wherein saidblade antenna element is positioned in the center of said bottom walland extending perpendicularly therefrom parallel to said side walls. 9.The cavity antenna of claim 8 wherein said blade antenna element ispositioned on a side wall and extending parallel to said bottom wall.10. The cavity antenna of claim 6 wherein said logic converterincludes;a connection to said transceiver for sensing the frequency oftransmission; a programmable-read-only-memory (PROM) circuit forproviding an instruction word as a function of a frequency sensed fromsaid connection; a control circuit connected to operate from aninstruction word from said PROM circuit and provide an output; and aline driver tuning circuit connected to said control circuit output totune said blade antenna element as a function of the control signalsfrom said control circuit.
 11. The cavity antenna of claim 10 whereinsaid conductive side and bottom walls form a rectangular box structure.12. The cavity antenna of claim 11 wherein said blade antenna element ispositioned in the center of said bottom wall and extendingperpendicularly therefrom parallel to said side walls.
 13. The cavityantenna of claim 12 wherein said blade antenna element is positioned ona side wall and extending parallel to said bottom wall.
 14. The cavityantenna of claim 10 wherein said conductive side walls are curved andsaid conductive bottom wall is flat and circular, and wherein said bladeantenna element is positioned in the center of said bottom wall andextends outwardly.
 15. The cavity antenna of claim 10 wherein saidconductive side walls are slanted inwardly and wherein said conductivebottom wall extends parallel to said open face and intersects saidslanted sidewalls, said blade antenna element being mounted on saidbottom wall and extends outwardly therefrom.
 16. The cavity antenna ofclaim 6 wherein said conductive side walls are curved and saidconductive bottom wall is flat and circular, and wherein said bladeantenna element is positioned in the center of said bottom wall andextends outwardly.
 17. The cavity antenna of claim 6 wherein saidconductive side walls are slanted inwardly and wherein said conductivebottom wall extends parallel to said open face and intersects saidslanted side walls, said blade antenna element being mounted on saidbottom wall and extends outwardly therefrom.